For safety reasons, it is important to prevent the buildup of dangerous pressures within battery casings by venting the battery when pressure begins to approach pressures which would otherwise cause explosion of the battery casing and consequent danger to people and property in the vicinity thereof. To this end, battery casing of lithium sulphur dioxide batteries which are hermetically sealed by welded joints have been provided with grooved thin wall sections which will rupture at an abnormally high pressure to relieve the pressure within the battery casing prior to the time the pressure reaches the point where the casing would generally explode to create hazardous conditions.
The present invention relates to an improvement in the type of vent disclosed in U.S. Pat. Nos. 4,109,135 and 3,918,610. The former patent discloses a casing pressure relieving vent in FIGS. 8 and 9 comprising a rib formed by deforming part of a grooved sheet metal end wall of a cylindrical casing to form an elongated casing projection or rib intersecting the pre-formed groove so that the groove extends transversely across the rib, that is down to and generally beyond the base of the rib. Internal casing pressure buildup places stresses on the rib which, when the venting pressure is reached, ruptures the rib along the groove. The grooves are stated to be formed by score lines which is presumed to involve cutting rather than coining the groove. However, it is not believed unique to coin rather than cut vent-forming grooves.
A variation of this grooved vent-forming rib is shown in the latter patent wherein a pair of such ribs are located between a pair of banana-shaped indentations extending inwardly from the general plane of the circular end wall of the casing and falling along a circle concentric with the circular configuration of the margins of the casing end wall. When the pressures inside the casing reaches an abnormally high but still safe value, the indentations begin to flatten. This places stresses on each grooved rib which fractures along the groove when the venting pressure is reached.
I discovered that certain advantages result by modifying the design of the vent-forming grooves in these ribs. It was apparently believed by the designers of these grooved vent-forming ribs that to be effective the grooves had to extend down to the base of the ribs. However, I discovered that such is not the case, and I also found that when the groove extends to the base of a rib it can cause undue weakening of the end wall causing it to rupture at the base of the rib in advance of the desired venting pressure.